With the stock Slackware 12.1 the USB drive should be recognized automatically. Before you manually mount:
Are rc.hald, rc.udev, and rc.messagebus all executable?
From the command line, before and after you insert a USB drive, as root run the lsusb and lsscsi commands. If the three mentioned services are running, you should see the drive added.
If the problem you are experiencing is KDE only, then in the Control Center, in KDE Components, Service Manager, ensure both KDED Media Manager and Media Notifier Daemon are enabled.